A conventional lens drive device includes an actuator made of shape-memory alloy. When receiving electric power, the actuator generates heat and then contracts, thereby moving the lens along a predetermined direction. The actuator then dissipates the heat in due course and expands, thereby moving the lens along the opposite direction.
An image-capturing device including the foregoing lens drive device controls a position of the lens with a resistance value varying in response to a deformation amount of the actuator, namely, the resistance value decreases in response to the contraction and increases in response to the expansion. (Refer to Patent Literature 1.)
In the foregoing image-capturing device, a voltage is applied to the actuator for generating heat when the resistance value exceeds a given value, and an application of the voltage is halted when the resistance value becomes smaller than the given value for inviting spontaneous heat dissipation. The voltage application and the halt of the application are repeated, so that the resistance value of the actuator can converge step by step on a given value. When the resistance value of the actuator agrees with the given value corresponding to a predetermined place of the lens, the image-capturing device recognizes that the lens is positioned at the predetermined place.
As discussed above, the resistance value of the actuator can be kept around a certain value; however, it is practically hard to maintain the resistance value rightly at the certain value. In other words, it is difficult for the image-capturing device to fix the lens at a certain place exactly. When the camera device recognizes that the lens is positioned at the predetermined place although the resistance value of the actuator is not stabilized yet, namely, the lens position is not yet fixed exactly or steadily, the data obtained through the lens can be short in stability or reliability.
Another image-capturing device including an actuator made of shape-memory alloy is disclosed in, e.g. Patent Literature 2, however, this literature is silent about how to control the lens.
Still another conventional image-capturing device formed of a lens movable along an optical axis and an image sensor, which receives light through the lens, is disclosed in, e.g. Patent Literature 3. This image-capturing device controls a position of the lens step by step in order to obtain data from respective fields provided in a focus region. A movement of the lens entails a decrease in a focus evaluation value, e.g. a decrease in brightness of image data, and the recognition of the decrease determines that the lens position in the event that the focus evaluation value starts falling is a target place.
The image-capturing device according to Patent Literature 3, however, has encountered a problem that the lens cannot be stopped at a desired target place, i.e. the place where the lens is supposed to stop. For movement of the lens produces a change in the focus evaluation value, so that a target place cannot be clearly recognized in such a case as: an image is shot around sunset, or an image is shot under poor sunlight, or multiple subjects should be shot.